Various instruments such as an engine cylinder, a container of a waterproof watch, a gas appliance and so forth are required to be completely free from a gas or liquid leak or to suppress leakage within a specified limit. To meet this requirement, such instruments or parts are checked for leakage during their manufacturing process.
As leakage inspection equipment for such a leakage test, pressure variation detecting type leakage inspection equipment has already been put to practical use. This leakage inspection equipment is roughly divided into two inspection systems, one of which applies a positive or negative fluid pressure to the interior of an article under inspection and detects whether a change in the pressure is within a prescribed range, thereby judging whether the article under inspection is non-defective or defective, and the other of which applies a positive or negative fluid pressure to each of the articles under inspection and a comparison tank and measures a variation in the differential pressure therebetween, thereby judging whether the article under inspection is non-defective or defective.
In either system, a fluid pressure is applied to the article under inspection, a variation in the pressure or differential pressure is monitored for a certain period of time after the fluid pressure reached a predetermined value, and the article being inspected is judged non-defective or defective depending upon whether the variation in the pressure or differential pressure is within a prescribed range. In such an inspection, air is usually employed as the fluid. In the case of using pneumatic pressure as the fluid pressure, an error is introduced into the measured pressure value or differential pressure due to various factors such as the temperature of the article under inspection, ambient temperature, humidity, water content adhering to the article under inspection and its slight deformation by pressure.
If the error value is always constant, no particular trouble occurs since it is necessary only to hold the reference value for judgement constant. Since the error contained in the pressure or differential pressure value is caused in each factor incurs a variation in the error value, and it is difficult to predict the variation. Accordingly, in the case of continuously performing the inspection, it is necessary that the reference value for judgement be frequently modified in response to variations in the error value. For such reasons, the prior art has the defect that the leakage inspection cannot be automated.
An object of the present invention is to provide pressure variation detecting type leakage inspection equipment in which the pressure or differential pressure value is automatically corrected to eliminate the error value, thereby making it possible to automatically perform an appropriate leakage inspection at all times.
Another object of the present invention is to provide pressure variation detecting type leakage inspection equipment which permits an appropriate leakage inspection from the initial stage of inspection.